Secondary batteries are often used as power supplies for driving portable electronic appliances or the like for the purpose of economy and resource saving and their use is remarkably expanding in these years. In accordance with size reduction and performance up-grade of electronic appliances, the batteries loaded therein are required to have a small size, light weight, and high capacity. On the other hand, conventional commonly used secondary batteries are lead-acid batteries and nickel-cadmium batteries. Recently non-aqueous lithium secondary batteries featuring a high energy density have been proposed and used in practice.
Lithium secondary batteries using non-aqueous electrolytes, however, have the problem that they have a low current density as compared with conventional secondary batteries using aqueous solution because the electrolyte using a non-aqueous solvent as an ion-conducting medium has a lower ion-conducting rate.
Proposals were made to overcome such problems, for example, by increasing the surface area of electrodes to increase their contact area with the electrolyte. More particularly, an electrode coating composition containing an active material and a polymer binder is applied to a current collector in the form of a thin metal foil to form a thin electrode layer thereon and a plurality of such coated foils are placed one on another or spirally wound with a separator interleaved therebetween. For example, JP-A 63-121260 describes a lithium secondary cell using a non-aqueous liquid electrolyte, LiCoO.sub.2 and/or LiNiO.sub.2 as a positive electrode, and carbon as a negative electrode. Each such electrode is an electrode layer of a microparticulate electrode active material bonded in a polymer-base binder.
However, some problems arise when an electrode layer is formed on a current collector typically in the form of a metal foil. Numerous repetition of charge-discharge cycles exacerbates the interfacial adhesion between the current collector and the electrode layer and lowers the discharge capacity of the electrodes, resulting in an insufficient cycle life. Fine particles of the electrode layer shed from the current collector can cause short-circuits.
One of the probable causes is that as the active material is expanded and contracted by doping and dedoping of lithium ions upon charging and discharging, defects occur at the electrode layer-current collector interface, active material-conductive material interface, active material-binder resin interface, etc. Also there arise problems that decomposition of polymer used as the binder by oxidation and reduction upon charging and discharging or swelling or dissolution of the polymer in non-aqueous electrolyte solution exacerbates the interfacial adhesion between the current collector and the electrode layer, and failure of conductor network by swelling of the polymer lowers the conductivity of the electrode layer.
The above-mentioned and other causes give rise to the problem of a short battery life in that repetition of charge-discharge cycles reduces the battery capacity.